(1) Field of the Invention
The present invention relates to the general technical field of inertial measurement systems and devices used in aviation and more particularly in aircraft, of the airplane, rotorcraft, or helicopter kinds.
The present invention relates more particularly to an inertial measurement device for mounting on board an aircraft.
Piloting and guiding an aircraft require the pilot to have an objective perception of the angles of inclination and of the heading of the aircraft.
In visual flight, the angles of inclination may be perceived directly by viewing the outside environment. The heading and variation in heading may also be estimated by landmarks. The use of a magnetic compass makes it possible to perform absolute estimation and is essential for most kinds of navigation.
When flying without visibility, special instruments are required to mitigate the lack of outside visual references. For angles of inclination and for heading, the instrument in question is referred to as an attitude and heading reference system (AHRS) and it is associated with a multifunction display (MFD) screen.
The manufacture of an AHRS requires gyros to be used that present accuracy (or drift) of the order of a few degrees per hour. Over the last two decades, fiber optic gyros (FOGs) have performed this function well, with drift often being less than one degree per hour.
With the arrival of so-called micro-electro mechanical systems (MEMS), gyros have become available at a price that is of the same order of magnitude as the price of a monolithic silicon integrated circuit (since the manufacturing methods are the same), i.e. a price that is extremely low.
Unfortunately, the bias (or drift) of such gyros from one run to another can often be measured in thousands of degrees per hour, whereas the stability of bias within a run is no better than about ten degrees per hour and is often as great as about one hundred degrees per hour under the effect of rapid changes of temperature. MEMS technology therefore misses the bias stability required for achieving the performance of high integrity AHRS instruments such as those required for operations under instrument flight rules (IFRs) by a factor of ten.
Of the three measurement axes, i.e. the yaw axis, the roll axis, and the pitching axis, it is the yaw axis that requires the greatest accuracy, since an AHRS instrument is often required to provide a heading, even in the absence of a magnetometer. Such a mode of operation is referred to as directional mode. Aircraft certification rules for this mode of operation require drift to be less than fifteen degrees per hour. Such performance is not achieved in reliable and repeatable manner using known gyros based on MEMS technology.
(2) Description of Related Art
By way of example, document U.S. Pat. No. 3,424,010 discloses an inertial measurement device having two gyros mounted at 90° and rotating relative to an axis perpendicular to the measurement plane for measuring two speeds of rotation given by the two gyros.
That device makes it possible to attenuate the measurement biases of two of the three angular speed components, but it is based on a speed of rotation that is fast since it is greater than the maximum angular speed of the carrier aircraft. Since such a device is not based on a constant inertial speed of rotation, its performance is mediocre. In addition, the high speed of rotation that is required gives rise to fast wear of the mechanisms and to high consumption of electricity.
To reduce the instabilities of the biases of MEMS gyros, document JP 02078961 (angular velocity detecting device) discloses devices in which bias variations are compensated by making temperature measurements in association with an interpolation table. Those devices present the drawback that is associated with the fact that the curve plotting the sensitivity of bias to temperature is itself not stable over time so it is necessary to perform periodic calibrations in order to mitigate the effects of aging.
Devices making use of temperature regulation are also known, e.g. from document FR 2 832 136 (a device for hermetically encapsulating a component that needs to be protected from all stresses). Such devices require large amounts of electrical power and suffer from a thermal stabilization time that is unacceptable on starting.
Also known, e.g. from document U.S. Pat. No. 7,066,004, is an inertial measurement device comprising an assembly of MEMS technology gyros and actuators for eliminating static or dynamic phase bias. Because of the difficulty of using MEMS technology to make bearings that provide freedom to perform a complete rotation about an axis, the device described in that document is based on rotary movements of limited amplitude that are intermittent and in alternating directions. The hinges allowing the alternating rotary movements are constituted by flexible elements. Those flexible elements, made of MEMS technology, are typically silicon structures of micrometer size, and they are very fragile in the event of an impact.